The use of a heating mechanism within projection exposure apparatuses is known from the publications US2005140947, US2008049202, US2010060988, U.S. Pat. No. 6,912,077, U.S. Pat. No. 6,504,597, U.S. Pat. No. 6,466,382, WO2009046895A1, EP1670041, and EP0823662.
Microlithography projection exposure apparatuses are used to produce microstructured components using a photolithographic method. A structured mask, the so-called reticle is illuminated with the aid of an exposure light source and illumination optics, and is imaged with the aid of projection optics on a substrate having a photosensitive layer. The exposure light source provides radiation which is passed to the illumination optics. The illumination optics is used for uniform illumination, with a predetermined angle-dependent intensity distribution, at the location of the structured mask. Various suitable optical elements are provided within the illumination optics for this purpose. The structured mask which has been illuminated in this way is imaged with the aid of the projection optics onto a photosensitive layer. The minimum structure width which can be imaged with the aid of projection optics such as these is governed, among other things, by the wavelength of the radiation used. In general, the shorter the wavelength of the radiation, the smaller are the structures which can be imaged with the aid of the projection optics. For this reason, an exposure light source is used which produces radiation in a first spectral range from 5 nm to 15 nm. The light within this spectral range is sometimes also called “used light” or “useful light” instead of exposure light. Since there are scarcely any transparent materials for this spectral range, mirrors are used as optical elements.
Microlithography projection exposure apparatuses are frequently operated as so-called scanners. This means that the reticle is moved through an object field in the form of a slot, along a scanning direction during a specific exposure duration, while the wafer is being moved appropriately on the image plane of the projection optics. The ratio of the speeds of the wafer to the reticle corresponds to the magnification of the projection optics between the recticle and the wafer, which is normally less than 1.
During operation of the exposure light source, all the mirrors in the EUV microlithography exposure apparatus have an intensity distribution applied to them in the first spectral range from 5 nm to 15 nm. Each of the mirrors has a first intensity distribution associated with it, which results from its position in the beam path and the specific embodiment of the exposure light source. Because the first intensity distribution associated with the mirrors is applied to the various mirrors, the mirrors are heated during operation of the exposure light source. In order to compensate for this, the mirrors are typically provided with appropriate cooling. Since the first intensity distribution associated with each mirror is constant over time in the first spectral range, this results in an equilibrium state after a specific time T1, in conjunction with the appropriate cooling, in which equilibrium state a first temperature distribution, which is constant over time, is present at each mirror. The temperature rise to the first temperature distribution leads to the optical characteristics of the mirrors changing. For example, the thermal expansion of the mirror substrates leads to a change in the radii of curvature of the mirrors. A change such as this is already taken into account in advance when calculating the optical characteristics of the illumination optics and the projection optics. However, this leads to the optical characteristics of the mirrors being optimal only when the mirrors have already reached their first temperature distribution. After the exposure light source has been switched on, it therefore takes until a time T1 before the optical system, including the exposure light source, illumination optics and projection optics, has reached its optimum state.